A common method for treatment of a bone fracture is to align portions of a bone that are separated by a fracture in a fixed spatial relationship. Generally, as the bone regenerates, the fractured portions of the bone will “knit,” and the fracture will be healed. Accordingly, simple fractures are often merely splinted while the fracture heals. In the case of serious fractures, medical devices may be employed to provide arthrodesis, or the surgical immobilization of a joint so that fractured bones grow together solidly.
A bracing device can be used in various arrangements to maintain the alignment of portions of a fractured bone. For example, a bracing device may be affixed to a portion of a bone with one or more surgical screws. Such use of surgical screws is a common practice in treatment of stress fractures in horses. A stress fracture is a prevalent form of injury to race horses that is not readily detectable by radiography. Often, stress fractures occur approximately ⅖ of the way from the knee to the fetlock joint of the horse's leg. The cannon bone may have a smaller diameter at this location than at other locations. A stress fracture is often detected as a density, or bump, overlaying the cortex of the cannon bone. The bump is due to new bone which is being laid down over the top of the fracture (callous formation) in an attempt by the horse's body to repair the injury.
Accordingly, prior art structures using surgical screws will, of necessity, cause contact of medical device components with soft tissue and muscles overlying the bone. As such, the requisite external device components, for example rods and plates, can often irritate the soft tissue and muscle. This irritation may cause either human or animal patients to suffer from chronic pain. However, devices requiring fixation to the bone do not provide a way to avoid contact with the surrounding tissue.
In this regard, U.S. Pat. No. 6,355,041 describes one prior art device for veterinary use for fetlock joint breakdown. A first end of the disclosed pin-plate device is a pin received in a bore formed in a center of a horse's third metacarpal bone, also known as the cannon bone. This is the horse's lower leg bone. A second end of the device is a plate affixed to a rear exterior surface of a first phalanx by surgical screws extending through the plate into the first phalanx. The first phalanx is a bone above the horse's hoof. The plate and the surgical screws contact surrounding tissue. Such contact is undesirable.
Another problem encountered with prior art bracing devices is “stress shielding.” Stress shielding is the loss of bone that occurs when stress is diverted from an area of bone. Bones tend to atrophy when they are unloaded. In natural body functioning, calcium is often lost from the bone where it is not needed for strength, resulting in a reduction in bone mass. Many prior art brace arrangements cause such stress shielding. For example, a steel rod brace inserted lengthwise in the center of a bone is many times stronger than the resulting bone surrounding it and thus removes some of the load from the bone. Consequently, there is an unequal sharing of the load between the steel rod and the bone, resulting in stress shielding of the bone. Such stress shielding is a major cause of failures in, for example, hip prosthesis surgery, as a steel rod inserted in the femur absorbs loading and leads to weakening of the femur.
There has been recognition in the art of the cause and effects of stress shielding. One suggested prior art technique to avoid this cause utilizes a hip prosthesis without a stem. Likewise, in the case of orthopedic fixation, it was noticed that screws that were significantly harder than bone could loosen. In addition to jeopardizing the healing process, this phenomenon could endanger adjacent anatomical structures. Accordingly, it has previously been suggested to use titanium screws since titanium has a level of flexibility reasonably close to that of bone and will transmit stress to a bone. However, there has not been great emphasis in providing natural loading of the bone with a device implanted therein. Limited progress has been made with respect to simulating normal load bearing in a surgically braced bone.
Another difficulty is that many previous schemes providing for arthrodesis do not allow normal patient functioning with a brace in place. This problem has been particularly difficult in veterinary practice since patients cannot be encouraged to stay still. One such example is equine fractures that are more severe than stress fractures. When a horse breaks the fetlock joint, or more particularly, the metacarpalphalangeal joint, for example, a breakdown in supporting structures of the fetlock joint leaves a horse unable to support its weight. In many such cases, the horse previously could not be provided with a suitable bracing structure to permit healing of the supporting structures while still enabling the horse to walk or stand normally. The horse would thus be unable to continue normal functioning. Traditionally, this type of injury has often resulted in the killing of the horse.
More recently, treatments have been provided to stabilize the fetlock joint through arthrodesis. However, complications have often followed arthrodesis. Typical complications include support limb laminitis, infection, implantation failure, and cast sores. Even when the treatment is ultimately successful, however, the horse typically does not regain normal leg function. This is because many of these previous treatments fuse the fetlock joint into an unnatural straight line, resulting in an extended limb length. This difference in limb length can cause a horse to overload its pastern and coffin joints, which are in the vicinity of the hoof. The overload can in turn lead to degenerative joint disease and pastern joint subluxation. The bones of the fetlock joint do not resume their original relative positions. Accordingly, while the horse may still be saved for breeding, it usually is unable to perform any other traditional functions.
In the treatment of humans, there are many previously known techniques for holding adjacent sections of a fractured bone together. One such technique is the insertion of pins. Often, however, additional surgery is needed for removal of these pins after a particular degree of bone regrowth has occurred. Another previous technique is the implantation of a plate to which bones must be affixed. However, the plate often provides a structure that does not duplicate the original shape of the bone. Casts are also commonly used to protect and stabilize fractures. Casts have the downside of greatly reducing the mobility of a patient, as well as causing sores and other irritation and great difficulty in bathing and other day-to-day tasks. Accordingly, it is highly desirable to avoid these prior shortcomings.
In this regard, U.S. Pat. No. 6,613,049 discloses a bone stabilizing frame system in which upper and lower clamping members are affixed to a bone on opposite sides of a fracture. External rods maintain the upper and lower clamps in a fixed spatial relationship. The size and location of the rods reduce the capability of a patient to function normally while healing in comparison to a stabilizing structure that would fit within a bone. However, this disclosed system has not been shown to be effective in practice.
Another example of an area in which difficulty has been encountered in tailoring available treatment procedures to avoid some of these traditionally encountered problems is spinal fusion. A common form of spine injury is herniation or other damage to intervertebral disks. Discs can compress against nerves in the spinal column and cause a high level of pain. Commonly, an entire vertebral disc is removed from between adjacent upper and lower vertebrae. The upper and lower vertebrae are fused to form a single spinal structure. Many forms of spinal fusion procedures have a low success rate, e.g., 40%. It is important to provide a procedure that fosters fusion between the vertebrae while maintaining a desired distance between the vertebrae adjacent the removed disc.
Accordingly, the current solutions for bracing bone fractures do not address in a flexible manner the need to provide relatively normal bone loading, minimal interference with muscle and soft tissue, and promotion of osteogenesis. The present subject matter addresses these needs.